VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY FACULTY OF CHEMICAL ENGINEERING Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) Mai Thanh Phong, Ph. FCE – HCMC University of Technology Chemical Reaction Engineering References 1. Octave Levenspiel, “Chemical Reaction Engineering”, John Wiley&Sons, 2002. Scot Foggler, “Elements of Chemical Reaction Engineering”,International students edition, 1989.Nauman, “Chemical Reactor Design”, John Wiley & sons, 1987.
Walas, “Reaction Kinetics for Chemical Engineers”,Int. Coulson & Richardsons, “Chemical Engineering – Vol 6”,Elsevier, 1979. Felder, “Elementary Principles of Chemical Processes”, John Wiley & sons, 2000. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 2 Chapter 1.
Introduction • Topic of the lecture „Chemical Reaction Engineering“ is the quantitative assessment of chemical reactions. The selection of suitable reactor types and their design will be discussed. • Reactor design uses information, knowledge, and experience from a variety of areas: thermodynamics, chemical kinetics, fluid mechanics, heat transfer, mass transfer, and economics. Chemical reaction engineering is the synthesis of all these factors with the aim of properly designing a chemical reactor.
• Thermodynamics tell us in which direction a reaction system will develop and how far it is from its equilibrium state. • Analyses of kinetics provide information about the rate with which the system will approach equilibrium. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 3 Chapter 1. Basic Parameter Description of the amount of a substance i: mi Number of moles: ni = Mi = molecular weight Mi ni Molar concentration: ci = V = volume V ni Mole fraction: xi = ∑ nj j Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 4 Chapter 1.
Introduction Progress of chemical reactions: ni 0 − ni Conversion: Xi = ni 0 ci 0 − ci If V = const: Xi = ci 0 ni − ni 0 Extent of reaction: ξ= νi Performance criteria: produced amount of product P Productivity: n& P = operating time Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 5 Chapter 1. Stoichiometry of chemical reactions: Stoichiometry is based on mass conservation and thus quantifies general laws that must be fulfilled during each chemical reaction. Starting point of a quantitative analysis is the following formulation of a chemical reaction: N ∑ν A = 0 i =1 i i This equation describes the change of the number of moles of N components A1, A2,. The νi are the stoichiometric coefficients of component i.
They have to be chosen in such a way that the moles of all elements involved in the chemical reaction remain constant. A convention is that reactants have negative stoichiometric coefficients and products have positive stoichiometric coefficients. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 6 Chapter 1. Introduction As an example the stoichiometric equation for the oxidation of carbon monoxide is given by: 2CO + O2 → 2CO2 with νCO = -2, vO2 = -1, vCO2 = 2 To calculate changes in the mole number of a component i due to reaction, the following balance has to be respected: ni = ni 0 + ν iξ From this equation results the important stoichiometric balance: ni 0 − ni Δni nk 0 − nk Δnk = = = νi νi νk νk Using the conversion X of a component k, the above equation becomes: νi ni = ni 0 − nk 0 X k νk Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 7 Chapter 1.
Chemical thermodynamics: Chemical thermodynamics deal with equilibrium states of reaction system. This Section will concentrate on the following two essential areas: a) The calculation of enthalpy changes connected with chemical reactions, and b) The calculation of equilibrium compositions of reacting systems.1 Enthalpy of reaction The change of enthalpy caused by a reaction is called reaction enthalpy ∆HR. This quantity can be calculated according to the following equation: N ΔH R = ∑ν i ΔH Fi i =1 ∆HFi is the enthalpy of formation of component i ∆HR < 0, the reaction is exothermic ∆HR > 0, the reaction is endothermic Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 8 Chapter 1. Introduction It is simple to calculate the reaction enthalpy at a certain standard state ∆HR0 from the corresponding standard enthalpies of formation ∆HFi0.
The standard enthalpies of formation are available from databases for P = P0 = 1 bar and T = T0 = 298 K. For pure elements like C, H2, O2,. The reaction enthalpy is a state variable. Thus, a change depends only on the Initial and the end state of the reaction and does not dependent on the reaction parthway.2 Temperature and pressure dependence of reaction enthalpy ⎛ ∂ΔH R ⎞ ⎛ ∂ΔH R ⎞ d (ΔH R ) = ⎜ ⎟ dP + ⎜ ⎟ dT ⎝ ∂P ⎠T ⎝ ∂T ⎠ P The pressure dependence is usually very small.
For ideal gas behaviour, the reaction enthalpy does not depend on pressure. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 9 Chapter 1. Introduction The correlation of reaction enthalpy and temperature is related to the isobaric heat capacities of all species involved in the considered reaction, cPi. N T ΔH R (T ) = ΔH R0 + ∑ν i ∫ c (T )dT Pi i =1 T = 298 K Assuming that the reactants and the products have different but temperature independent heat capacities, the temperarue dependence of the reaction enthalpy can be estimated as follows: ΔH R (T ) = ΔH R0 + (T − T0 )(cP ,products − cP ,reactants ) Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 10 Chapter 1.3 Chemical equilibrium • Chemical reactions approach to an equilibrium, when the product and reactant concentrations do not change anymore.
• A reacting system is in chemical equilibrium if the reaction rates of the forward and backward reactions are equal. • The basic quantity required to indentify the equilibrium state is the Gibbs free enthalpy of reaction GR. • The change of this quantity becomes zero when the equilibrium is reached (i. dGR = 0) For constant pressure and temperature, the change of free Gibbs enthalpy of reaction can be described as follows: N ⎛ dGR ⎞ N dGR = ∑ν i μi dξ or ⎜⎜ ⎟⎟ = ∑ν i μi i =1 ⎝ dξ ⎠T , P i =1 Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 11 Chapter 1.
Introduction In Figure 1-1 is shown the course of free Gibbs enthalpy of reaction as a function of the extent of reaction. Free Gibbs enthalpy The equilibrium is reached when the free ⎛ ∂GR ⎞ ⎛ ∂GR ⎞ Gibbs enthalpy of reaction is minimum. ⎜⎜ ⎟⎟ < 0 ⎜⎜ ⎟⎟ > 0 ⎝ ∂ξ ⎠T , P ⎝ ∂ξ ⎠T , P Thus, for the chemical equilibrium: ⎛ dGR ⎞ ⎜⎜ ⎟⎟ = 0 ⎝ dξ ⎠T , P ⎛ ∂GR ⎞ ⎜⎜ ⎟⎟ = 0 Or dGR=0 (or in an integrated form: ∆GR = 0) ⎝ ∂ξ ⎠T , P Thus, the equilibrium is characterized by: ξ N Fig. 1-1: Changing of free Gibbs enthalpy ∑ν μ = 0 i =1 i i for a chemical reaction Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 12 Chapter 1.1 Reation free Gibbs enthalpy, ∆GR N ΔG = ∑ν i ΔGFi0 0 R i =1 ΔGFi0 free Gibbs energy of formation Relation between ∆GR and ∆HR ( d ΔGR0 T ) ΔH R0 =− 2 dT T Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 13 Chapter 1.2 Equilibrium constant and temperature dependence Relationship between the free Gibss enthalpy and the equilibrium constant: ΔGR0 (T ) = − RT ln K ⎛ ΔGR0 ⎞ K = exp⎜⎜ − ⎟⎟ ⎝ RT ⎠ Van‘t Hoff equation describing the temperature dependence of the equilibrium constant: d (ln K ) ΔH R0 =− dT RT 2 For a small temperature range, ∆HR is constant, thus: ΔH R0 ⎛ 1 1 ⎞ ln K (T2 ) = ln K (T1 ) − ⎜⎜ − ⎟⎟ R ⎝ T2 T1 ⎠ Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 14 Chapter 1.
Reaction rate Based on unit volume of reacting fluid: 1 1 dni • VR is volume of the reaction mixture r= • ni is mole number of component i ν i VR dt • t is reaction time If VR is constant: 1 dci • ci is molar concentration of component i r= ν i dt Based on unit mass of solid in solid-liquid systems: 1 1 dni r= • W is mass of solid ν i W dt Based on unit solid surface of solid-liquid or solid-gas systems: 1 1 dni r= • S is solid surface area ν i S dt Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 15 Chapter 1. Standard Reactors To carry out chemical reactions discontinuously operated reactors or continuously operated reactors can be used. • Discontinuously: more frequently applied to produce fine chemicals • Continuously: more advantageous for the production of larger amounts of bulk chemicals. To study the different behavior of these types of reactors another important criterion serves to distinguish two limiting cases: mixed flow and plug flow behavior For theoretical studies and to compare the different reactors, four different ideal reactors can be defined using the above classification: a) Batch Reactor (BR, perfectly mixed, discontinuous operation): Features: • All components are in the reactor before the reaction starts • Composition changes with time • Composition throughout the reactor is uniform Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 16 Chapter 1.: • Simple, flexible, high conversion… Disadv.: • Dead times for charging, discharging, cleaning,… • Difficult to control and automate •… BR are applied in particular for: • Relatively slow reactions • Slightly exothermic reactions Areas of application for BR are: • Reactions in pharmaceutical industry • Polymerisation reactions • Dye production • Speciality chemicals Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 17 Chapter 1.
Introduction b) Semi-batch Reactor (SBR): perfectly mixed, semi continuous operation Features: • One reactant is introduced first and then the second is dosed in a controlled manner. • Composition changes with time • Composition throughout the reactor is uniform Adv.: • Controlled reaction rate and heat generation • .: • Same as BR • More complicated than BR •… Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 18 Chapter 1. Introduction c) Continuously Stirred Tank Reactor (CSTR): perfectly mixed, continuous operation Features: A,B A,B,products • Reactants are continuously introduced, products (+ unconverted reactants) are continuously withdrawn • Composition does not change with time • Composition throughout the reactor is uniform Adv.: • Controlled heat generation • Easy to control and automate • No dead times • Constant product quality,.: • Complicated • Can become unstable • Large investmnent cost,. Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 19 Chapter 1.
Introduction d) Plug Flow Tubular Reactor (PFTR): no mixing, continuous operation A, B tubular reactor A, B, products Features: • Composition varies from point to point along a flow path Adv.: • High conversion • Easy to automate • No dead times • Better to cool (compare to stirred tanks) •… Disadv.: • Complicated • Danger of “hot spot” •… Mai Thanh Phong - HCMUT Chemical Reaction Engineering 3-Feb-09 20 VIETNAM NATIONAL UNIVERSITY – HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY FACULTY OF CHEMICAL ENGINEERING Chemical Reaction Engineering (Homogeneous Reactions in Ideal Reactors) Mai Thanh Phong, Ph. FCEMai ThanhUniversity – HCMC Phong - HCMUT of Technology Chemical Reaction Chemical Engineering Reaction Engineering 3-Feb-09 21 Chapter 2. Interpretation of Batch Reactor Data 1. Rates of reaction 1.
Description of reaction rates Reaction rates depend usually in a complex manner on the concentrations, the temperature and often on the effect introduced by catalysts: r = f ( ci ,T, catalyst) The order of a reaction is related to the concentration dependence.